A mold provided by pairing a cavity plate and a core plate is employed for injection molding using various materials, such as a plastic (synthetic resin), a ceramic, a rubber, a glass and liquid crystal, or for die casting using a variety of materials, such as aluminum, zinc, tin and copper. When a molten material, such as plastic or metal, is injected into an enclosed space (hereinafter also referred to as a cavity), and is thereafter cooled in a predetermined manner, a finished product having a desired shape and structure is obtained. Generally, when a plastic, a ceramic, a rubber, a glass or a metal, that is, a raw material, is heated until it reaches an appropriate molding condition, a gas consonant with the material component is generated. The types and amounts of gases generated differ, depending on the heating temperatures, the material types, indirect materials to be added, etc., and also differ in accordance with mold cavity volumes.
Fine finishing is performed for the mold cavity, to improve the surface property and the external appearance of a finished product, and is also performed for the contact surfaces of the cavity plate and the core plate to ensure an airtight plate juncture. An elastic packing may also be positioned between the cavity and core plates, at their contact surfaces, to more simply provide an airtight juncture. Within the airtight mold cavity, gas components, such as those generated by a molten material and residual air, are present, as previously described. These gas components, which failed to be exhausted by an impelling force when a molten material was injected, prevent the molten material from spreading throughout the interior of the cavity, so that molding failures, such as short shots or surface irregularities, tend to occur. Further, when there is a gap in the juncture between the plate contact surfaces that permits gas components to be freely exhausted from the mold cavity, the molten material could enter the gap and create mold flashing or blowholes (unevenness) on the surface of the molded product, thereby degrading the product quality.
According to patent document 1, which is conventional art that provides, for the removal of the adverse affects of gas enclosed in the mold cavity, a gas vent path 11 that communicates with a gas vent hole 10, formed near the distal end of the mold cavity. A structure disclosed releases gas by opening and closing, using a slidable core member 12, the portion that connects the gas vent hole 10 and the gas vent path 11. In this case, a timer and another control mechanism are employed to selectively open and close the channel using the core member 12. The operating timing is determined based on the setup of a controller that is selected while taking into account the flow state of a molten material and other relevant conditions.
However, the flow state of a molten resin greatly differs, depending, for example, on the type of thermoplastic resin that is used as the molding material, the use of either a single resin or an alloy prepared by mixing a plurality of resins, the mold structure, the size of a finished product, the structure of the mold cavity, and whether or not an insert is used. For determining the setups for the timer and the other control means, the repetitious employment of trial and error is required by actually flowing molten resin, and this process imposes a greater load on operators responsible for molding to prevent the occurrence of defects. Further, when employing in die casting a specific type of metal or alloy, since the flow velocity of a molten material is remarkably high because of its low viscosity, adequate control for opening and closing of the channel is difficult.
Patent document 2 discloses that slide slots are formed from the surface of a cavity to the inside of a mold, and that a moving member is provided that is movable along the slide slots in the axial direction to form a gas release groove. The objective is the opening and closing of gas passage means that employs the forward or backward movement of the moving member, which occurs upon the contact of the leading end of a molten resin, to permit communication between the interior and exterior of the cavity via the gas release groove. However, detailed machining of a mold is requisite, and machining, maintenance, checking, repairing, etc., for the mold are complicated.
Patent document 3 discloses a structure wherein a gas vent valve is provided near the resin injecting portion at the bottom end of a cavity, and wherein, at the initial processing time, the gas vent valve 7 is opened and gas is released, and subsequently, when a pressure sensor located near a gate has detected a pressure rise in the resin, the gas vent valve is closed. According to this conventional art, in the initial period for injecting the molten resin, the gas vent valve is open, and when the pressure is increased in association with the flow state of the resin, the gas vent valve is closed to prevent leakage of the molten resin. However, for this conventional art, as well as for patent document 1, control means for opening or closing the gas vent valve, based on signals received from the pressure sensor, is additionally required.
Patent Document 1: JP-A-9-277310
Patent Document 2: JP-A-2000-15668
Patent Document 3: JP-A-2003-170479